I sense the "Grissom and Sara in Paris" two-parter and the motion picture are two separate items.
I don't see how Grissom and Sara being in the CSI movie would interfere with the rest of the cast. CSI has done ensemble episodes before where everyone gets their moment to shine, why can't they do it in a 2 hour+ movie? Yes, they're not CSIs anymore, but they could be old friends coming back, hostages, witnesess, consultors... they could even be the victims who validate the CSI rule: you have sex, you die. They could have a raunchy sex scene and then be killed ("Who killed The Grissoms?" would be a topic big enough to make a movie about it, huh?). Or they could cross to the dark side and become serial killers (hey! one never knows!). But seriously, they can totally have Grissom and Sara there and have them as a couple (because that's how they will be) without the movie being "the GSR show".
Given WP's words, I expect the movie to be very gory and/or creepy, full of politically incorrect stuff and packed with action, and a big intelectuall puzzle with some kick-ass villain. I really, really hope the "everybody needs to be included" includes the labrats, specially Bobby and Archie, and I really, really hope the director of the movie is one of the home directors of CSI.